With the trends toward higher densities and higher integration degrees in LSIs and toward higher densities and larger display sizes in liquid-crystal display panels, the yields and reliability of products have come to be considerably influenced by whether the foreign matters (particles) present on the surfaces of wafers or glass substrates can be removed completely and efficiency. In the case of a semiconductor wafer, for example, the steps of film formation, resist application, exposure, development, etching, resist removal, cleaning, etc. are repeated for the formation of circuit patterns. Any foreign matter present on a semiconductor wafer on its circuit pattern side is causative of circuit breakage or short-circuiting in the formation of a next circuit. Any foreign matter present on the back side of a semiconductor wafer (the side opposite to the circuit pattern side) not only is causative of a failure of focusing in the exposure step in circuit formation, but is causative of circuit breakage or short-circuiting when transferred to the surface of the adjacent wafer.
Various techniques have hence been proposed and put to practical use in order to heighten the level of cleanliness in LSI production processes and the level of wafer cleaning techniques. In particular, the cleaning steps account for about 30% of all steps of a process, and an improvement of the cleaning steps is a key to a higher yield and higher reliability. The same applies in fields which mostly relate to photofabrication such as processes for producing liquid-crystal display panels, plasma display panels (PDP), hard disks, optical disks, magnetic heads, and optical parts/lenses and processes for producing other precision electronic parts. However, with the recent trend toward finer patterns, the problems of cleaning techniques used in prior art processes have become serious.
Cleaning techniques include wet cleaning (with ultrapure water, chemical liquids, etc.) and dry cleaning (with UV ozone, O.sub.2 plasma, etc.). In general, wet cleaning is frequently used from the standpoint of a good balance between cleaning ability and economy. A problem of wet cleaning is the re-adhesion of a foreign matter (particles) once removed by cleaning. In particular, any foreign matter adherent to the back surface of a wafer (on the side opposite to the circuit pattern side) is a potent contaminant. Another problem is pollution in a drying step, which is necessary for wet cleaning.
Attention is focused on techniques of smoothing wafer surfaces for heightening the degree of integration in semiconductors. Among such techniques is CMP (chemical mechanical polishing), which has been put to practical use. In the CMP of wafers, since a slurry of an abrasive material is used, abrasive particles and polishing dust particles adhere to the wafers in large amounts. These adherent particles should be removed, which procedure is currently being conducted by wet cleaning.
Dry cleaning (with UV ozone, O.sub.2 plasma, etc.) is coming to be increasingly employed as a cleaning technique which mitigates or eliminates the drawbacks of wet cleaning. Although dry cleaning has advantages of the diminution of foreign-matter re-adhesion and no need of a drying step, it was found that the ability of dry cleaning to remove foreign matters (particles) is insufficient. In order to eliminate these problems, methods for foreign-matter removal with a pressure-sensitive adhesive or pressure-sensitive adhesive tape were proposed.
The foreign-matter removal with a pressure-sensitive adhesive is a technique which comprises applying a solution of a pressure-sensitive adhesive to a surface having a foreign matter adherent thereto, drying the solution, and then stripping the pressure-sensitive adhesive to thereby bond the adherent foreign matter to the pressure-sensitive adhesive surface and remove the same.
The foreign-matter removal with a pressure-sensitive adhesive tape is a technique which comprises applying a pressure-sensitive adhesive tape comprising a substrate and a pressure-sensitive adhesive layer to a surface having a foreign matter adherent thereto and then stripping the tape to thereby bond the adherent foreign matter to the surface of the pressure-sensitive adhesive layer of the tape and remove the same. Advantages of the foreign-matter removal with a pressure-sensitive adhesive or pressure-sensitive adhesive tape are that it is free from the problems of the ordinary techniques for foreign-matter removal described above, and that removal of a foreign matter (particles) by dry cleaning is possible.
Methods for removing an adherent foreign matter from the surface of a silicon wafer or glass substrate using the technique described above are proposed in, e.g., JP-A-48-35771, JP-A-53-92665, and JP-A-1-135574. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") Since these proposed methods are a kind of dry cleaning, the problem of foreign-matter re-adhesion in wet cleaning and the problem of pollution in a drying step can be avoided. In addition, the proposed methods are expected to have the higher ability to remove foreign matters as compared with other dry cleaning techniques using UV ozone, O.sub.2 plasma, etc.
However, the methods proposed in the references cited above have problems that when the pressure-sensitive adhesive used is susceptible to plastic deformation and has high adhesive force, in stripping the pressure-sensitive adhesive or pressure-sensitive adhesive tape, there are some problems in handling properties and wafer breakage. In order to avoid these problems, JP-A-7-142440 proposes a method which comprises applying a pressure-sensitive adhesive tape employing a pressure-sensitive adhesive which cures upon ultraviolet irradiation, irradiating the tape with ultraviolet to reduce the adhesive force of the pressure-sensitive adhesive, and then stripping the tape. This prior art method has a drawback that if the pressure-sensitive adhesive used is hard and has low adhesive force, it is difficult to sufficiently bond and fix a foreign matter to the surface of the pressure-sensitive adhesive. Another drawback thereof is that the application and stripping of the pressure-sensitive adhesive tape may result in static buildup, which leads to the electrostatic breakdown of the semiconductor wafer. An effective measure against static buildup in production processes is indispensable especially to CCDs (charge coupled devices) because CCDs have a lower electrostatic withstand voltage than general MOSs and LSIs. Moreover, there is a possibility that the static buildup might cause adhesion of suspended particles and re-adhesion of a foreign matter, namely the pressure-sensitive adhesive tape itself might be a pollution source.